1. Field of the Invention
The present invention relates to a circuit board having a via hole formed by filling a through-hole in an interlayer insulation film with an electrode, the through-hole having an inverted-trapezoidal cross-section, which is suitable for use in the microwave or millimeter-wavebands. The present invention also relates to a manufacturing method for such a circuit board.
2. Description of the Related Art
In recent wireless communication systems, miniaturization, cost reduction, and higher performance of RF devices for use in the microwave or millimeter-wave bands are increasingly demanded. For example, efficient transmission lines having low transmission losses and electrode materials having low resistances are desired.
Interlayer insulation films between connections are usually desired to be made of a dielectric material having a low relative dielectric constant and a low dielectric dissipation factor.
To meet these needs, a multilayer circuit board using a low-resistance material such as Au, Cu, Ag, or Al as the electrode material and an organic resin having a low relative dielectric constant and a low dielectric dissipation factor such as polyimides, epoxies, benzocyclobutene, bismaleimide triazines, or other suitable material as a dielectric material has been developed.
Referring to FIG. 5, one conventional method of making a via hole in an interlayer insulating film of the multilayer circuit board uses photolithography. The method makes the through-hole for forming a via hole by using a photomask 50 including a light-shielding pattern 51 for forming a through-hole (a through-hole for forming a via hole). As shown in FIG. 5, the way of using photolithography is that firstly, the photomask 50 is placed on a photosensitive resin film (not shown), secondly, the photosensitive resin film is exposed by using ultraviolet light, thirdly, the resin film is developed by a solvent, lastly, the unexposed portions of the resin film are dissolved and removed.
However, as shown in FIG. 6, when the interlayer insulation film 63 covering a lower conductor connection 62 on the surface of a substrate 61 has a large thickness, and if a through-hole for forming a via hole is made by using above process, a tilt of inner walls 64a of the through-hole 64 which is formed in the interlayer insulation film 63 is insufficient. Therefore, the tilt of the through-hole 64 is approximately perpendicular. When an upper conductor connection 65 is formed on the interlayer insulation film 63, as shown in FIG. 7, the upper conductor connection 65 is not connected to the lower conductor connection 62, which is formed on the substrate 61, because edges 64b of the through-hole 64 disconnect the upper conductor connection 65.
In other words, when forming the upper conductor connection 65, the through-hole for making a via hole must have an inverted-trapezoidal cross-section and inner walls which are suitably tilted in order to form a defect-free uniform metallic film on the inner walls 64a of the through-hole 64.
Examples of known methods for forming a through-hole for defining a via hole, the through-hole having an inverted-trapezoidal cross-section and inner walls which are suitably tilted, include:
(1) a method for forming a micro pattern that is smaller than the resolution limit of the photosensitive resin film, which is the precursor film of the interlayer insulation film;
(2) a method for forming a pattern that is smaller than the resolution limit of the photosensitive resin film at a through-hole formation-pattern region on an exposure mask and adjusting the layout of the pattern; and
(3) a method for adjusting the number, the arrangement, the shape, and other characteristics of the openings in the pattern.
In method (1), for example, the film is a positive photosensitive resin film, which has the exposed portions being dissolved and removed during development. The film is exposed at a low exposure level so that the film is developed to a depth corresponding to the exposure level. According to method (1), the inner walls of the through-hole for making the via hole can be sufficiently tilted, but control of the tilt angle is practically impossible.
Methods (2) and (3) can control the shape of the through-hole for making the via hole and the tilt angle of the inner walls. However, in order to control the tilt angle of the inner walls to a desired angle, test runs must be performed using various types of photomasks. This causes problems including increased costs for making the various types of photomasks and increased time to evaluate the test results.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method for manufacturing a circuit board including a through-hole having an inverted-trapezoidal cross-section and sloped inner wall stilted at a desired tilt angle without requiring complex processes or any increase in cost.
In addition, preferred embodiments of the present invention provide a circuit board in which an upper connection conductor is reliably connected to a lower connection conductor by using the manufacturing method of the present invention.
According to one preferred embodiment of the present invention, a method for manufacturing a circuit board includes the steps of forming a lower conductor connection on a substrate, applying a photosensitive resin on the lower conductor connection and drying the photosensitive resin to form a photosensitive resin film, exposing the photosensitive resin film via a photomask for patterning, and developing and drying the photosensitive resin film so as to form an insulating resin film including a through-hole for forming a via hole, the through-hole having an inverted-trapezoidal cross-section taken in the direction of the substrate thickness and sloped inner walls tilted at a tilt angle xcex8 with respect to the substrate, and forming an upper conductor connection on the insulating resin film so as to connect the upper conductor connection to the lower conductor connection through the through-hole in the insulating resin film. The photomask has a via hole pattern including a light-shielding pattern corresponding to the bottom of the through-hole, and pluralities of light-shielding strips and translucent strips which are arranged alternately and substantially parallel to one another, the pluralities of light-shielding strips and translucent strips corresponding to the sloped inner walls of the through-hole. The tilt angle xcex8, which is determined using a polynomial approximation of degree n:       f    ⁡          (      s      )        =      θ    =          φ      ⁢                        ∑                      k            =            0                    n                ⁢                              C            k                    ⁢                      s            k                              
wherein s is the width of the light-shielding strips and xcfx86 is a constant relating to the exposure conditions, is in the range of about 0.17 rad less than xcex8 less than about 1.40 rad.
According to the above-described method, a through-hole having an inverted-trapezoidal cross-section and a sloped inner wall tilted at an tilt angle xcex8 of about 0.17 rad less than xcex8 less than about 1.40 rad can be efficiently formed in a circuit board.
Moreover, because the tilt angle xcex8 is determined by the nth degree function described above, the tilt angle xcex8 can be adjusted to any desired angle within the range of about 0.17 rad less than xcex8 less than about 1.40 rad by simply changing the width of the light-shielding strips. Thus, a circuit board in which an upper conductor connection is connected to a lower conductor connection can be reliably manufactured.
The tilt angle xcex8 is adjusted within the range of about 0.17 rad less than xcex8 less than about 1.40 rad to secure electrical connection between the lower and upper conductor layers without inhibiting the miniaturization of devices. At an angle exceeding about 1.40 rad, the sloped inner wall is insufficiently tilted, and the connection between the lower and upper conductor layers breaks, as described in the related art shown in FIGS. 6 and 7. At an angle of less than about 0.17 rad, the connection is secured, but the diameter of the via hole is excessively large, thereby inhibiting miniaturization of devices.
Preferably, each of the upper conductor connection and the lower conductor connection includes at least one material selected from the group consisting of Cu, Au, Ag, Al, Ni, Ti, Cr, NiCr, Nb, V, Ta, W, Pt, and Mo so as to reliably manufacture circuit boards having all the required characteristics.
Preferably, the insulating resin film includes at least one resin selected from polyimides, epoxies, benzocyclobutene, bismaleimide triazines, acrylics, and cyclic olefins. With these materials, an insulating resin film having required insulation characteristics can be reliably formed.
Preferably, in the step of forming an upper conductor connection on the insulating resin film, the upper conductor connection is formed on the insulating resin film by one of vapor deposition, sputtering, and plating so as to reliably manufacture a circuit board having high connection reliability at the via hole.
Preferably, the translucent strips in the photomask are aligned substantially parallel to the top edge of the sloped inner wall, and the widths of the translucent strips are gradually increased from that of the strip corresponding to the bottom of the sloped inner wall to that of the strip corresponding to the top of the sloped inner wall so that the exposure level of the sloped inner wall is gradually increased from the bottom to the top of the sloped inner wall. With such a photomask, a through-hole having an inverted-trapezoidal cross-section and a sloped inner wall tilted at a desired tilt angle can be reliably formed.
Preferably, the width of each of the translucent strips is in the range of about xc2xc to about {fraction (1/20)} of the thickness of the insulating resin layer and the interval between the translucent strips is in the range of about xc2xc to about {fraction (1/20)} of the thickness of the insulating resin layer so as to make a through-hole having an inverted-trapezoidal cross-section and a sloped inner wall tilted at a desired tilt angle.
Preferably, the number of translucent strips in the photomask is 2 to 10. Using such a simple photomask, a circuit board having a through-hole having an inverted-trapezoidal cross-section and a sloped inner wall tilted at a tilt angle xcex8 in the range of about 0.17 rad less than xcex8 less than about 1.40 rad can be reliably formed.
Another preferred embodiment of the present invention provides a circuit board including a substrate, a lower conduct or connection, an insulating resin layer including a through-hole for defining a via hole, and an upper conductor connection. The lower conductor connection and the upper conductor connection are connected to each other through the through-hole. The through-hole has an inverted-trapezoidal cross-section, and a tilt angle xcex8 of a sloped inner wall of the through-hole with respect to the substrate surface is in the range of about 0.17 rad less than xcex8 less than about 1.40 rad.
A circuit board having the above-described unique structure has an upper conductor layer and a lower conductor layer securely connected to each other through a via hole because the tilt angle xcex8 of the sloped inner wall of the through-hole is in the range of about 0.17 rad less than xcex8 less than about 1.40 rad.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following description of preferred embodiments thereof with reference to the attached drawings.